The present invention relates to an optoelectronic device with an integrated optical guide and photodetector. It is used in optoelectronics and in particular optical telecommunications, where the device according to the invention can constitute the optical receiver of a heterodyne reception circuit.
The device according to the invention can also be used in optical interconnections between high speed electronic chips with a high degree of integration, or between computers, or within computers.
An optoelectronic device with an integrated optical guide and photodetector is shown in FIG. 1, with three slightly different embodiments. In all these embodiments the device comprises, on a semiconductor substrate S and in succession: a first lower confinement layer CiC made from a first material having a first index (n1), a second guide layer CG made from a second material and having a second index (n2) higher than the first (n1) and a third upper confinement layer CsC made from a third material with a third index (n3) lower than that of the second material (n2).
This structure is completed by a detector D positioned at different locations as a function of the embodiment, either in the lower part of the substrate (part a in FIG. 1), a mirror M at 45.degree. reflecting the light from the guide layer downwards, or at the end of the guide layer (part b), or above the guide layer (part c) for an evanescent wave operation.
The device according to the invention relates to the third embodiment, i.e. that which will be described in greater detail hereinafter.
An integrated detector--guide device with coupling by evanescent wave is e.g. described in the article by R. J. DERI et al entitled: "Integrated Waveguide/Photodiodes with Vertical Impedance Matching", published in "Proceedings of the 1989 IEDM", Washington D. C., December, 1989.
Although satisfactory in certain respects, all evanescent wave coupling devices have the disadvantage of requiring a significant length for the detector, in order to obtain a total absorption of the optical radiation. However, this necessity is contray to one of the sought objectives, namely the reduction of the response time of the device, which would lead to the reduction of the surface of the detector and consequently its length.
In order to better understand the origin of this problem and the solutions proposed by the prior art, reference can be made to the attached FIGS. 2 and 3, where two known embodiments are shown in greater detail. In these drawings, part a is a plan view, part b a section at the entrance of the detector and part c a section at the exit of the detector. The circular or elongated spot FL represents a section of the light beam propagating in the device.
The least absorbing devices have, between the guide layer CG and the absorbent material Ab, an upper confinement layer CsC in the form of a guide for maintaining the transverse confinement on an approximately 3 .mu.m band (cf. FIG. 3). However, the separation between the guide and the absorbent reduces the absorption by evaescence.
A structure of this type is described by J. A. CAVAILLES et al in the article entitled "Integration of Detectors with GaInAsP/InP Carrier Depletion Optical Switches", published in "Electronics Letters", Oct. 11, 1990, vol. 26, no. 21, p. 1783.
Moreover, M. C. AMMAN in an article entitled "Analysis of a PIN Photodiode with Integrated Waveguide", published in "Electronics Letters", Aug. 13, 1987, vol. 23, no. 17, p. 895 sought the optimum thickness for the absorbent leading to a maximum absorption. However, this constraint makes it technically difficult to obtain the detector and only modifies by a factor of 2 the absorption coefficient.
Another problem caused by these devices is associated with the detector width, i.e. the dimension perpendicular to the guide (dimension visible in parts b and c of FIGS. 2 and 3). It is possible to make a distinction between two detector types, depending on whether the width is significant (FIG. 2) or not (FIG. 3). The wide detectors are used for avoiding lateral losses due to the natural widening of the beam. For example, a width of 32 .mu.m is used by R. J. DERI et al, referred to hereinbefore, for 5 to 7 .mu.m guide widths. In this case, for a detector length of 190 .mu.m, the beam widens and passes from 7 .mu.m at the entrance to the detector to 15 .mu.m at the exit, as shown in parts b and c of FIG. 2. Thus, the detector width would have to exceed 20 .mu.m to bring about total absorption.
A limited width of 3 .mu.m (for a length of 100 .mu.m) was used by J. A. CAVAILLES et al, referred to hereinbefore, with a beam guided laterally (i.e. without widening) by an InP ribbon layer etched with a thickness of 0.2 .mu.m above the guide layer (cf. parts b and c of FIG. 3). However, the upper InP confinement layer between the absorbent and the guide reduces the absorption.
R. J. DERI et al propose shortening the absorption length by introducing an anti-reflection layer between the guide layer CG and the absorbent Ab. The index and thickness of this layer must be chosen so as to reduce the reflection at the interface between the guide layer and the absorbent layer. Taking 3.162 for the index of the guide and 3.53 for the index of the absorbent, a good index choice consists of taking an intermediate value of e.g. 3.22. The maximum absorption value corresponds to an anti-reflection layer thickness between 0.5 and 0.6 .mu.m. The value chosen by R. J. DERI et al is 0.55 .mu.m. The structure then has the appearance shown in FIG. 4, where the anti-reflection layer carries the reference CaR.
For a given anti-reflection layer thickness, a simple absorption calculation gives a 90% absorption for a length of 190 .mu.m in agreement with the value chosen experimentally by R. J. DERI et al. However, said layer increases the thickness of the intrinsic zone and therefore increases the transit time of the charge carriers created by the absorbed beam. Only a photoconductive diode or a M.S.M. diode makes it possible to reduce the response time, because the electric field in this case is applied to the surface. Although the detector length is reduced by the use of said anti-reflection layer, the surface of the detector still remains relatively large.